Many synthetic thermoplastic polymers are poor conductors, or nonconductors, of electricity. A consequence is that after the polymer or a blend of the polymer and other ingredients has been fashioned into a molded article, coating, film or fiber, electrostatic charges tend to accumulate on the surface and are not freely dissipated. This is especially characteristic of polymers having surface resistivities greater than 10.sup.12 (ohms or ohms/square), many of which are commercially important. Electrostatic charges can accumulate on the surface of these polymeric materials to levels equivalent to 20,000 to 30,000 volts. Even with lower levels of static build-up, however, many undesirable effects can still occur. Contact with synthetic materials in automobile seat covers, floor rugs, clothing, and so forth may create a high static charge on a person which, when subsequently discharged by contact with a grounded object, causes an unpleasant sensation of shock. The accompanying spark, moreover, can create a serious hazard in flammable or explosive atmospheres, such as found in hospital operating rooms (anesthesia gases) and underground excavations for the mining of ore.
The problem is especially acute in the electronics industry, where relatively low static charges can result in a catastrophic failure of sensitive micro-electronics devices, and in the business machine industry, where paper jam-ups in photocopiers are often directly attributable to the accumulation of electrostatic charges as the copy paper passes over plastic platens and guides.
Various ways have been proposed in the past for treating at least some of these polymers to make them more dissipative of surface charges. Some involve modification of the polymer itself. For instance, Ohya, et al., in U.S. Pat. No. 4,384,078, have proposed that a more static resistant polymeric material can be obtained by graft polymerizing a vinyl or vinylidene monomer, such as sodium styrene sulfonate, onto a rubbery copolymer of an alkylene oxide and a conjugated diene. The resulting product is said to be blendable with other thermoplastic resins and utilizable in conjunction with conventional antistatic agents. Borman, on the other hand, has disclosed in U.S. Pat. No. 3,259,520 that polyphenylene oxide resins can be altered to be antistatic by forming certain ionic derivatives through nuclear substitution with groups such as sulfonate groups.
Still other methods involve the formation of blends of antistatic agents with the polymer. Castro, et al., in U.S. Pat. No. 4,210,556, teach that a liquid ethoxylated amine such as N,N-bis(2-hydroxyethyl) alkenyl or a mixture of alkenyl and alkenyl amines can be admixed with a polymer, for example, a polyolefin or polyphenylene oxide, to form a homogeneous liquid which can then be cooled to a solid antistatic agent. The solid can be blended into a polymer to impart antistatic properties.
Baron, et al., in U.S. Pat. No. 3,933,779, disclose that certain bis-ethoxylated quaternary ammonium salts of paratoluene sulfonic acid are useful as antistatic agents for various synthetic polymers, including polystyrenes, polyesters, polyamides, polycarbonates, polyolefins, and ABS resins.
Abolins and Katchman have found that an antistatic agent based on a mixture of triethanolamine, toluene sulfonic acid and sodium lauryl sulfate, is an effective additive for polyphenylene ether resins and blends. This discovery is described in U.S. Pat. No. 4,123,475.
Japan Pat. No. 47-22474 discloses that certain metal salts of a sulphonated vinyl aromatic compounds, for example, sulphonated polystyrene, are useful as antistatic agents for polymeric materials.
U.S. Pat. No. 4,341,882 describes blends of polyphenylene ether, polystyrene and an antistatic agent which can be a styrene-allyl alcohol copolymer, an anionically polymerized poly(ethylene oxide) and a combination of both.
More recently, Luxon has shown in U.S. Pat. No. 4,384,063 that the antistatic behavior of N,N-bis-(2-hydroxyethyl-N-octyl-N-methyl ammonium para toluene sulfonate (also identified in the above mentioned Baron, et al. patent as methyl-octyl-bis(2-hydroxyethyl) ammonium para-toluene sulfonate), is enhanced in a polyphenylene ether resin blend when used in conjunction with a small amount of a polyethylene glycol ester.
Polymer blends modified by the addition of antistatic compounds based on amines often suffer from certain deleterious effects, however. The additive can often be removed from the surface of a molded part by simply rinsing with water or a non-aqueous solvent, and this detracts from the surface antistatic behavior. In the case of some polymers where migration of the additive to the surface occurs, the antistatic effect may return after only several hours or days. For other polymers, however, examples of which are polyphenylene ether resins and polystyrenes, the antistatic behavior recovers much more slowly, and may not fully return for several months. Moreover, many antistatic amines have poor compatibility with the polymer, a result of which is that only small amounts of the additive will be tolerated. The latter phenomenon can present the following dilemma. On the one hand, the use of concentrations of additive above the threshold of compatibility may lead to processing difficulties during injection molding. Typically, this is manifested by extruder screw slippage as a result of lubrication from excess (incompatible) amounts of the additive, and by erratic molding cycle times. On the other hand, the relatively small amounts of additive dictated by these processing requirements necessarily limits the antistatic performance of the molded article.